The present invention relates to an apparatus for the thermal measurement of a texture of a porous body, called a thermoporometer. Porous bodies are used in numerous branches of industry and in general whenever an insoluble solid phase has to be intimately contacted with a fluid. This is the case with hydrocarbon synthesis or cracking catalysts, fillers introduced into elastomers or paints, pigments or absorbents used for the purification or chemical analysis (molecular sieves, absorbents for chromatography), hydraulic binders (cement), powders used in the preparation of emulsions (photographic emulsions, emulsions of products for agricultural treatments). Porous bodies are also used in separative methods such as ultrafiltration.
The methods used for studying the texture of a porous body are dependent on the size of the pores. For a diameter exceeding roughly 50 nanometers, it is necessary to use optical microscopic analysis methods. The invention relates to the study of the texture of porous bodies, whereof the pore radius is less than approximately 150 nanometers. Several methods for measuring the porosity of such porous bodies or substances are already known.
Thus, there are direct observation methods such as selectronic microscopy. This method makes it possible to directly observe the structure of the porous body, but is unfortunately difficult to perform in the case of mesoporous bodies, i.e. in which the average pore radius is between 2 and 50 nanometers.
There are also indirect methods based on the detection of capillary phenomena occurring in the porous bodies. These methods deduce the characteristics of the porous bodies either from the mechanical equilibrium of the surfaces (mercury porometry), or from the liquid - vapour thermodynamic equilibrium conditions of a condensate held within the porous body (Barrett, Joyner and Halenda or B.J.H. method).
The mercury porometry method described more particularly in "Powder Technology", vol. 29 - 1, May-June 1981, published by ElsevierSequoia, SA, Lausanne makes it possible to determine the porous distribution and deduce the specific surface. It consists of injecting mercury under high pressure into the porous body.
This method consists of measuring the variations in the apparent volume of the mercury-porous solid system subject to increasing pressures. From this is then deduced the distribution of the pore radii of the curve giving the pressure applied as a function of the injected mercury volume. The radii obtained are in fact the radii of the accesses to the porous cavities and not the radii of the actual porous cavities.
This method theoretically makes it possible to reach pores with a radius of 3 nanometers on using pressures close to 2.5 kbars. However, at these pressures, the texture of the porous body can be disturbed by crushing before the intrusion.
The B.J.H. method is a method for calculating the porous distribution based on the stepwise analysis of the desorption branch of the adsorption - desorption isotherm of a condensable vapour (nitrogen, argon, etc.). This method is described in detail in "The determination of pore volume and area distributions in porous substances. Computations from nitrogen isotherms" published in the Journal of the American Chemical Society, vol. 73, 1951, pp. 373-380.
As for the mercury porometry method, the disadvantage of this method is that it only gives information on the size of the access orifices to the pores and not on the size of the actual pores.
There is also an indirect method, called thermoporometry, described in particularly in the journal Thermochemica acta, no. 21, 1977, pp. 59 to 88. This method consists of a thermodynamic study of the liquid-solid and vapour phases of a fluid, which completely saturates a porous body. This study supplies a first relation making possible to calculate the size of the pores and subsequently to shape as determined the shape factor based upon the radii of the pores and in which the change of state has taken place from the variation between the temperature of the normal triple point of the free condensate and the temperature at which this change of state occurs in the pores. A second relation between the temperature and the apparent solidification or freezing energy measured in the pores makes it possible to calculate the corresponding porous volume.
With the aid of these relations and the solidification or freezing thermogram, which is the recording of the power given off by the freezing or solidification of the capillary condensate during a linear temperature drop it is possible to determine the distribution curve of the pore radii and the corresponding porous volumes.
This method is based on the fact that the equilibrium conditions of the solid, liquid and gaseous phases of a pure substance in a highly divided state are a function of the curvature of the interfaces. In the case of a fluid contained in a porous body, the curvature of the solid - liquid interface is imposed by the pores size. The freezing or solidification temperature is consequently different in each pore of the material.
The solidification thermogram of a known condensate in an unknown porous body consequently permits the determination of the size of the pores by the measurement of the solidification temperature or freezing point and the volume of said pores by measuring the state change energy. Thus, this method gives the real size of the pores and not that of the access orifices thereof, as in the case of the B.J.H. and mercury porometry methods.
More specifically, in the thermoporometry method, at each temperature freezing takes place in the pores of a given size. Knowing the apparent solidification of freezing energy. the volume of these pores is determined by a calorimetric measurement of the energy released at this temperature. Thus, the pore radius distribution curve is directly deduced from the solidification of freezing thermogram.
The known thermal measuring equipment realizing the aforementioned method are generally calorimeters designed for making precise heat quantity measurements. On such equipment, the temperature measurement is of a secondary and not very accurate nature.
However, this temperature measurement is vital in thermoporometry, because it forms the basis for the method for calculating the pore radii. In the known equipment, the difficulties of checking or controlling the temperature consequently make it impossible to obviate the surface of the fluid in which the porous body is immersed during the temperature drop. As a function of the fluids used, this supercooling range has a varying significance. It can reach 10.degree. C., and more in the case of water. Thus, the supercooling range limits the use of the solidification or freezing thermogram.